1. Field of the Invention
The present invention relates to a thin film magnetic head and magnetic recording and reproducing apparatus.
2. Discussion of Background
A thin film magnetic head in the prior art is disclosed in, for example, U.S. Pat. No. 4,856,181. Its basic structure includes rail portions provided at intervals on the surface of the slider, constituted by a ceramic structure, that faces opposite the medium . The surfaces of the rail portions are air-bearing surfaces with a high degree of flatness. At the end of each rail portion in the direction of air flow, a thin film magnetic transducer element is mounted. The thin film magnetic transducer element is a thin film element formed by a process similar to that of IC manufacturing technology and it is formed by integrating a lower magnetic film, a gap film, an upper magnetic film, a coil film, an insulating film, a protective film and the like. The lower magnetic film and the upper magnetic film have a lower pole piece and an upper pole piece respectively that face opposite each other across the gap film at their front end surfaces. The structure of these pole pieces and the gap film constitutes a transducing gap.
Such a thin film magnetic head is mainly used in a magnetic recording and reproducing apparatus that is used in storage system for computers and the binary data to be handled are modulated in correspondence with magnetized and non magnetized regions, or the direction of magnetization of the medium. Typical methods of modulation include the peak detection method and the PRML (partial response maximum likelihood) method which combines the regenerated waveform equalization processing based upon the partial response method, and the signal decoding system based upon the Viterbi decoding algorithm. Of these, PRML has been attracting much attention as a signal processing method that can support high-density recording and is disclosed in U.S. Pat. No. 4,571,734, U S. Pat. No. 4,644,564 and U.S. Pat. No. 4,707,681.
Since the thickness of the tips at the end surfaces of the pole pieces of a thin film magnetic head is approximately consistent at the thickness of the magnetic film which is formed during the manufacturing process, a sub pulse, attributable to a pseudo gap, is generated at the position that corresponds to the end edge of the pole piece. This sub pulse is characteristically manifested in a thin film magnetic head in which the pole pieces are very thin. If a sub pulse occurs, it interferes with the main pulse, changing the main pulse. When a sub pulse interferes with and changes the main pulse, it becomes difficult to use the PRML method.
The concept for inhibiting this sub pulse is disclosed in Japanese Examined Patent Publication No. 29090/1978, Japanese Unexamined Patent Publication No. 287411/1987, Japanese Unexamined Patent Publication No. 91812/1988 and Japanese Unexamined Patent Publication No. 103410/1988. In Japanese Examined Patent Publication No. 29090/1978, a method is disclosed wherein after forming the upper magnetic film or the lower magnetic film, a third magnetic film is sputtered on at least one of these magnetic films. It is then etched to ensure that the entirety of the magnetic films will not lie parallel to the gap film. Japanese Unexamined Patent Publication No. 287411/1987 discloses a method whereby the entirety of both the upper magnetic film and the lower magnetic film are formed in such a way that their edges will lie non parallel to each other. Japanese Unexamined Patent Publication No. 91812/1988 discloses a method whereby the effect of the pseudo gap is reduced by lowering the magnetic permeability .mu. on the outside of the pole pieces.
In another facet of the prior art, Japanese Unexamined Patent Publication No. 13210/1992 discloses a method whereby the sub pulse is suppressed by changing the thickness of at least one of the pole pieces in such a manner that the surface on the opposite side of the gap film will form a curved surface. Then, in Japanese Unexamined Patent Publication No. 146510/1992, a method is disclosed whereby the sub pulse is suppressed with stages provided on the opposite side of the gap film in the pole pieces. Japanese Unexamined Patent Publication No. 163707/1992 and Japanese Unexamined Patent Publication No. 163708/1992 disclose a method whereby the sub pulse is suppressed by structuring the pole pieces with a plurality of layers of magnetic film integrated with one another to control the saturation magnetic flux density of the magnetic films.
Additionally, Japanese Unexamined Patent Publication No. 263603/1991 discloses a method whereby the contour effect is suppressed by forming the core end in a shape that is unparallel with the gap. The publication details that the contour effect can be even more effectively reduced if the shape of the core end is set in such a manner that the difference .DELTA.P between the maximum pole length Pmax and the minimum pole length Pmin of the core end will fall within the, range of approximately 1/3 to 2 times the recording wavelength.
However, with the prior art technology described above, it has not yet been possible to suppress the sub pulse to such an extent that PRML can be adopted. Although the technology disclosed in Japanese Unexamined Patent Publication No. 263603/1991 is effective in reducing the sub pulse, the publication does not disclose a means for reducing the sub pulses that are generated at the lower pole piece and the upper pole piece to a similar degree so that they will be well balanced. Because of this, even when the sub pulse generated at the upper pole piece has been flattened to the extent whereby the partial response method could be adopted, the sub pulse at the lower pole piece is not reduced as much as the one at the upper pole piece. Therefore, adoption of PRML is still quite difficult.